To reduce fuel consumption, harmful emissions and noise levels in vehicular traffic, start-stop systems, by means of which the internal combustion engine is automatically switched off when the vehicle is at rest and automatically re-started when a wish to begin moving again is recognized, are becoming increasingly important. In order to enable this function in the case of an automatic transmission in which a hydraulic pump is driven by the internal combustion engine to supply pressure and cooling oil to the shifting clutches and shifting brakes of the transmission, the shifting clutches or shifting brakes required for a starting gear must already be at their contact point or even engaged when the internal combustion engine is stopped, or at least at a time close to the re-starting of the internal combustion engine. In this way it can be ensured that the conventional hydraulic pump is able, during an engine start that takes only a short time so that then only a small volume of hydraulic fluid is needed, to produce a sufficient oil pressure for quick torque transmission at the shifting elements. However, according to the prior art the shifting pistons of the shifting elements, which are usually mechanically spring-loaded in the disengagement direction, move to the disengaged position as soon as the hydraulic pressure falls when the motor and pump stop. With the consequently produced air-gap clearance, it is true that any non-actuated disk packets can move in consort, with low drag torques, but bridging the no-load stroke distance and the associated quantity of pressure fluid demanded before torque transmission in the starting gear can begin, results in a perceptible delay in starting which impairs the comfort of the start-stop operation.
From DE 10 2006 012 838 A1 for example, it is known, when the engine stops, to drive an electric auxiliary pump having a permanent power uptake, which at least maintains a hydraulic supply pressure for the shifting elements involved in starting. Furthermore, from WO 2007/118 500 A1 it is known to use additionally incorporated impulse storage devices which are charged with fluid during driving operation and, when the engine is re-started, ejecting this fluid charge into the hydraulic system to assist a rapid pressure build-up by the hydraulic pump. In addition, from DE 10 2006 014 759 A1 a hydraulic control device is known, by means of which the shifting elements that are relevant for a start-stop function are prevented from disengaging when the engine is off by one-way valves which restrict the back-flow of hydraulic fluid.
DE 199 32 614 A1 describes a hydraulically actuated piston for engaging and disengaging a disk clutch of a variable-speed automatic transmission. Associated with the clutch piston is a diaphragm plate which, together with the piston, forms an additional pressure space that acts in opposition to a piston space. When the pressure in the two pressure spaces is the same, the resultant pressure force depends only on the difference between the two pressure surface areas. Thus, by controlling the piston area difference, different shifting moments compared with the effect of the entire piston surface can be produced. In particular, with clutches which, as such, have to produce high static holding torques in shifting processes which only require a low torque, such as in shifts during coasting, low shifting torques can be achieved by means of the piston area difference, which attenuate comfort-impairing shift jerks.
DE 101 31 816 A1 discloses an assembly of two disk brakes. Each of the two disk brakes has a pressure space in which a piston is acted upon by a hydraulic fluid which, for its part, actuates a secondary piston and the secondary piston in turn acts upon a disk packet of the disk brakes. On the rear side of the secondary piston extended radially outward is arranged in each case a second pressure space radially above the disk packet, i.e. on a diameter larger than that of the disk packet. The second pressure spaces have in each case a fluid inlet through which they can be acted upon by a hydraulic fluid under pressure in order to push back the respective secondary piston and with it the main piston, whereby the first pressure space is emptied. The second pressure spaces can be actively controlled by electro-hydraulic or electro-pneumatic pressure modulation as regulated restoring means. The engagement and disengagement of the disk brakes can in each case take place by controlling the pressure difference in the two pressure spaces in order to achieve sensitive shifting with small and very different shifting moments.
DE 10 2007 003 922 A1 describes a clutch piston of a rotating disk clutch in an automatic transmission of a motor vehicle, in which a pressure compensation space is designed to form a dynamic restoring means. The pressure compensation space compensates for the displacement forces on the piston the are usually produced by oil rotating in a piston space due to centrifugal force, and in addition produces a resultant restoring force that replaces a conventional spring. In contrast to a restoring spring, however, the restoring force of the pressure compensation space only acts on the piston when the clutch is rotating. If the drive motor stops, so too does rotation of the piston. Consequently, despite the falling hydraulic pressure the piston remains in a forward end position since there is no longer a restoring force, i.e. it is nearly in the engaged position. Then, when the engine is re-started, only a small oil volume has to be replaced, whereby the necessary shifting pressure for a starting process in start-stop operation is provided in an acceptably short time.